Had Boeing chosen all nickel-cadmium batteries instead of lithium-ion batteries for its 787 Dreamliner, how much additional weight would have been added to the plane?

One nickel-metal hydride battery weighs about 100 pounds, or about 40 pounds more than a lithium-ion battery, Donald Sadoway, a battery expert and materials chemistry professor at MIT, told the Atlanta Journal-Constitution in an email. There are two lithium ion batteries — one for power backup and one for engine ignition — on every 787 Dreamliner, Sadoway said.

"That means a difference of about 40 pounds per battery, which is less than the luggage allowance for a single passenger," Sadoway wrote. He wrote that nickel-metal hydride batteries are used in the Toyota Prius.

All 50 of Boeing's 787s were grounded earlier this month during an investigation to find out why lithium-ion batteries failed on two planes. One caused a fire on a 787 and another forced an emergency landing.

U.S. tops in guns per person

Is there any other nation in the world with a citizenry in possession of as many privately owned firearms per capita as the United States? If not, any even come close?

Before we could get to this question, our curious reader tracked down a good source for an answer and shared it.

A recent Washington Post story cited a survey conducted in 2007 by researchers from the University of Geneva that places the United States far ahead of other nations in the average number of firearms per 100 civilians.

According to the study, titled the Small Arms Survey, 89 of every 100 Americans had a firearm. The survey estimated that the 314 million Americans had 270 million firearms.

Suicide rates are lower in winter, particularly in December, than other times of the year, according to John L. McIntosh, the associate vice chancellor for academic affairs and professor of psychology at Indiana University South Bend, who has studied and written extensively about suicide. One caveat, he writes: Recent data is tougher to analyze because of confidentiality issues.

There were 38,364 suicides in the United States in 2010, according to the Centers for Disease Control and Prevention, averaging 105 a day. There were an average of 98.2 a day in December that year, the lowest month of the year. The highest average for a month was July, with 111.3 a day, followed by August with 111.2.